This invention relates to cements and to improved concrete compositions comprising said cements in combination with a set-retarding compressive strength increasing agent, an aggregate mixture, optional air-entraining admixtures, sufficient water to provide desirable workability and, optionally, other concrete additives. More particularly, the compositions herein are characterized by increased compressive strength afforded by the presence of cellulose acetate sulfate therein.
Cement is one of the most widely used materials in construction. Concrete is composed of cement; "aggregate" which comprises sand, gravel or broken stone, etc.; and various optional additives which modify or impart desirable characteristics or properties to the concrete. In general, concrete comprises about one volume of cement to two volumes of sand and four volumes of stone. These ratios can be varied, depending on the strength and other structural parameters desired.
Cement, itself, is a powdery complex calcined mixture of various inorganic calcium, aluminate and silicate compounds which can be made into a paste with water and which will cure or "set" into a solid mass when allowed to stand. In general terms, concrete is cement-plus-aggregate. Hydraulic cements are most commonly used to prepare concrete. The use of hydraulic cements of one form or another has been known since the Mesopotamian era and the age of early Egypt. Various limes were also employed by the early Greeks and later by the Romans as the principal component in cements and concrete.
It has now been found that the use of cellulose acetate sulfate in cement and concrete (or mortar) compositions provides a significant improvement in the compressive strength properties thereof after setting.
It is well-known that hydraulic cement mixtures (and concrete prepared therefrom) require excess water to be workable. This excess of water results in an overall decrease in strength. Thus, concrete compositions which are workable, yet which require only a small amount of water, are desirable. This desirable "water reduction" attribute is secured by means of the cellulose acetate sulfate additives herein. The present additives serve a two-fold purpose: (1) to retard the setting rate, thus prolonging the use life; (2) to lubricate the cement and aggregate particles so that they exhibit improved flow characteristics and require lesser amounts of water during the mixing and molding operation, thereby enhancing the strength of the cured cement or concrete.
The modification of cements and concretes by means of various starches and cellulosics to achieve slow setting or increased workability at lower water concentrations is well-known.
U.S. Pat. No. 2,795,508, Kaveler, Sulfoalkyl Cellulose Ethers and Their Salts as Hydraulic Natural Cement Set Retarders, June 11, 1957, discloses a cement composition having set-retarding properties containing a sulfoalkyl cellulosic ether salt compound as an additive.
U.S. Pat. No. 2,995,189, Cutforth, Hydraulic Cements having an Extended Thickening Time, Methods of Making the Same, and Processes Employing the Same, Aug. 8, 1961, relates to cement compositions containing a set-retarder additive composition comprising a mixture of non-acetylated acid cellulose sulfate and its salts and carboxymethyl hydroxyethyl cellulose mixed ether and its salts.
U.S. Pat. No. 3,753,748, Martin, Hydraulic Cement Retarder Composition, Aug. 21, 1973, relates to hydraulic cement slurries containing as a set-retarding additive a synergistic mixture of an alkali metal, alkaline earth metal, or alkali metal-alkaline earth metal salt of a lignosulfonic acid in combination with boric acid or a borate.
U.S. Pat. No. 2,880,096, Hurely, Cement Compositions and Process of Cementing Wells, Mar. 31, 1959, discloses a hydraulic cement admixture containing carboxyalkyl hydroxyethyl cellulose mixed ethers and salts thereof and an alkali metal aluminate as a composition which reduces the water needed in the slurry while accelerating setting time.
U.S. Pat. No. 3,788,869, Batdorf, et al., Concrete Compositions or Mixes and Additives Therefor, Jan. 29, 1974, relates to readily pumpable concrete compositions containing cellulosics.
As can be seen from the foregoing, there is a continuing search for methods to improve cement and concrete compositions using various additives which provide set modifying benefits and which impart improved strength thereto.
It is an object of the present invention to provide improved cement containing an additive which increases the compressive strength of the hardened cement and concrete (or mortar) made therefrom.
Another object of this invention is to provide a cement additive which retards the set rate of the cement, or concrete made therefrom, without adversely affecting the properties thereof.
A further object is to provide low water content cement or concrete compositions which are readily workable and which exhibit enhanced strength and durability on curing.
In a preferred mode, the present invention encompasses the use of an air-entraining additive which imparts improved freeze/thaw characteristics and additional workability to the aforesaid improved concrete mixtures.
These and other objects are obtained herein as will be seen from the following disclosures.